The present invention relates to aluminum alloys, and more particularly to a 2000 series alloy of the aluminum-copper-magnesium type characterized by high strength, very high fatigue resistance, and high fracture toughness.
A significant economic factor in operating aircraft today is the cost of fuel. As a consequence, aircraft designers and manufacturers are constantly striving to improve overall fuel efficiency. One way to increase fuel efficiency, as well as overall airplane performance, is to reduce the structural weight of the airplane. Since aluminum alloys are used in a large number of structural components of most aircraft, significant efforts have been expended to develop aluminum alloys that have higher strength-to-density ratios than the alloys in current use, while maintaining the same or higher fracture toughness, fatigue resistance, and corrosion resistance.
For example, one alloy currently used on the lower wing skins of some commercial jet aircraft is alloy 2024 in the T351 temper. Alloy 2024-T351 has a relatively high strength-to-density ratio and exhibits good fracture toughness, good fatigue properties, and adequate corrosion resistance. Another currently available alloy sometimes used on commercial jet aircraft for similar applications is alloy 7075-T651. Alloy 7075-T651 is stronger than alloy 2024-T351; however, alloy 7075-T651 is inferior to alloy 2024-T351 in fracture toughness and fatigue resistance. Thus, the higher strength-to-density ratio of alloy 7075-T651 often cannot be used advantageously without sacrificing fracture toughness and/or fatigue performance of the component on which it is desired to use the alloy. Likewise, other currently available alloys in their various tempers; for example, alloys 7475-T651, -T7651, and -T7351; 7050-T7651 and -T73651; and 2024-T851; although sometimes exhibiting good strength or fracture toughness properties and/or high resistance to stress-corrosion cracking and exfoliation corrosion, do not offer the combination of improved strength, improved fracture toughness, and improved fatigue properties over alloy 2024-T351. Thus, with currently available alloys in various tempers, it is usually impossible to achieve weight savings in aircraft structural components presently fabricated from alloy 2024-T351 if fracture toughness, fatigue resistance, and corrosion resistance must be maintained at or above the current levels.
It is therefore an object of the present invention to provide an aluminum alloy for use in structural components of aircraft that has a higher strength-to-density ratio than the currently available alloy 2024-T351, and additionally which has improved fatigue and fracture toughness characteristics over alloy 2024-T351. It is further object of the present invention to maintain stress-corrosion resistance and exfoliation-corrosion resistance at a level approximately equivalent to or better than that of alloy 2024-T351.